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Date: Tue, 10 Aug 2021 18:30:21 +0800 From: Lai Jiangshan <laijs@...ux.alibaba.com> To: Paolo Bonzini <pbonzini@...hat.com>, Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org Cc: Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org Subject: Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT On 2021/8/10 18:07, Paolo Bonzini wrote: > On 09/08/21 19:43, Lai Jiangshan wrote: >> From: Lai Jiangshan <laijs@...ux.alibaba.com> >> >> Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug >> registers") allows the guest accessing to DRs without exiting when >> KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized >> on entry to the guest---including DR6 that was not synced before the commit. >> >> But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT, >> but also when KVM_DEBUGREG_BP_ENABLED. The second case is unnecessary >> and just leads to a more case which leaks stale DR6 to the host which has >> to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put(). >> >> We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT, >> so that we can fine-grain control the cases when we need to reset it >> which is done in later patch. >> >> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com> >> --- >> arch/x86/kvm/x86.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index ad47a09ce307..d2aa49722064 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >> set_debugreg(vcpu->arch.eff_db[1], 1); >> set_debugreg(vcpu->arch.eff_db[2], 2); >> set_debugreg(vcpu->arch.eff_db[3], 3); >> - set_debugreg(vcpu->arch.dr6, 6); >> + /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */ >> + if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT) >> + set_debugreg(vcpu->arch.dr6, 6); >> } else if (unlikely(hw_breakpoint_active())) { >> set_debugreg(0, 7); >> } >> > > Even better, this should be moved to vmx.c's vcpu_enter_guest. This > matches the handling in svm.c: > > /* > * Run with all-zero DR6 unless needed, so that we can get the exact cause > * of a #DB. > */ > if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) > svm_set_dr6(svm, vcpu->arch.dr6); > else > svm_set_dr6(svm, DR6_ACTIVE_LOW); > > That is, > > KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT > Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug > registers") allows the guest accessing to DRs without exiting when > KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized > on entry to the guest---including DR6 that was not synced before the commit. > But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT, > but also when KVM_DEBUGREG_BP_ENABLED. The second case is unnecessary > and just leads to a more case which leaks stale DR6 to the host which has > to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put(). > Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters > on VMX because SVM always uses the DR6 value from the VMCB. So move this > line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT. > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index ae8e62df16dd..21a3ef3012cf 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu) > vmx->loaded_vmcs->host_state.cr4 = cr4; > } > > + /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */ > + if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT) > + set_debugreg(vcpu->arch.dr6, 6); I also noticed the related code in svm.c, but I refrained myself to add a new branch in vmx_vcpu_run(). But after I see you put the code of resetting dr6 in vmx_sync_dirty_debug_regs(), the whole solution is much clean and better. And if any chance you are also concern about the additional branch, could you add a new callback to set dr6 and call the callback from x86.c when KVM_DEBUGREG_WONT_EXIT. The possible implementation of the callback: for vmx: set_debugreg(vcpu->arch.dr6, 6); for svm: svm_set_dr6(svm, vcpu->arch.dr6); and always do svm_set_dr6(svm, DR6_ACTIVE_LOW); at the end of the svm_handle_exit(). Thanks Lai > + > /* When single-stepping over STI and MOV SS, we must clear the > * corresponding interruptibility bits in the guest state. Otherwise > * vmentry fails as it then expects bit 14 (BS) in pending debug > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index a111899ab2b4..fbc536b21585 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > set_debugreg(vcpu->arch.eff_db[1], 1); > set_debugreg(vcpu->arch.eff_db[2], 2); > set_debugreg(vcpu->arch.eff_db[3], 3); > - set_debugreg(vcpu->arch.dr6, 6); > } else if (unlikely(hw_breakpoint_active())) { > set_debugreg(0, 7); > } > > Paolo
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